Liquid crystal displays (LCD) have been widely applied in electrical products, such as digital watches, calculator, etc. for a long time. Moreover, with the advance of techniques for manufacture and design, thin film transistor-liquid crystal display (TFT-LCD) has been introduced into portable computers, personal digital assistants, and color televisions, as well as gradually replacing the CRT used for conventional display. The demands for TFT-LCD tend to be large in scale.
The typical size of a mask used in the photolithography process is much less than the size of the panel of a liquid crystal display. Therefore, a same photolithography process step has to apply to different regions in the panel. However, even though the process parameter value about machine can be controlled in the same situation, it is impossible to control the process parameter value about environment to keep the same. In other words, a same photolithography process applied to different regions in a panel can have different parameter values. Therefore, a shot mura defect is often generated between any two adjacent masks.
Accordingly, the structure of the conventional thin film transistor is illustrated in FIG. 1A, in which a gate electrode 204 and the storage capacitor electrode 206 are formed on a glass substrate 200. An insulating layer 208 is formed on the substrate 200 to cover the gate electrode 204 and the storage capacitor electrode 206. An amorphous silicon (a-silicon) layer 210 is formed above the insulating layer 208 and the gate electrode 204, and an n+ amorphous silicon layer 212 is deposited on the top surface of the a-silicon layer 210. In additional, a source/drain electrode structure 214 is formed above the n+ a-silicon layer 212. The data lines structure 216 is defined over the insulating layer 208, too, when forming the source/drain electrode structure 214. Moreover, a passivation layer 218 is formed on the top surface of glass substrate 200 to cover the a-silicon layer 210, the source/drain electrode structure 214 and the data lines structure 216. A contact hole 220 is formed on the passivation layer 218 to expose the top surface of the source/drain electrode structure 214. Then, an ITO layer 222 is formed on the passivation layer 218 to connect the source/drain electrode structure 214.
The diffusion capacitor (Cgs) is the capacitor between the gate and source/drain electrodes. The storage capacitor (CST) is the capacitor between the ITO layer 222 and the storage capacitor electrode 206. The capacitor (CLC) is the pixel capacitor. FIG. 1B shows a waveform diagram for driving the thin film transistor LCD. The pixel capacitor CLC and the storage capacitor (CST) are charged to the voltage value, VP, when the scan line scans the thin film transistor at a given time T1. The thin film transistor is turned off at the non-selective time T2. The pixel capacitor is maintained by the maintenance capacitor. However, the instant the thin film transistor is turned off, the voltage value (VP) may fall by ΔV. The ΔV is related to the diffusion capacitor (Cgs) between the gate and source electrodes, pixel capacitor (CLC) and the storage capacitor (CST). The ΔV value is shown as follows:ΔV=(Vgh−VgL)×Cgs/(Cgs+CLC+CST)  (1)
The Vgh is the high voltage providing to the gate electrode. The VgL is the low voltage providing to the gate electrode. The gate electrode 204, the storage capacitor electrode 206, the ITO layer 222 and the a-silicon layer 210 are respectively formed by using different photolithography processes. Therefore, when a large scale LCD panel is divided into several regions for respectively performing photolithography process, an abrupt image difference is generated if the process parameters between regions are different. For example, the ΔV value is different if a misalignment exists between regions.
Referring to FIG. 2, it illustrates a schematic diagram of an in-plane switching (IPS) mode liquid crystal display. In general, IPS technology enlarges the viewing angle of a liquid crystal display. The liquid crystal molecule in a typical liquid crystal display is rotated up or rotated down to display an image. However, the liquid crystal molecule in the IPS mode liquid crystal display is rotated in a plane to display an image. According to the IPS mode liquid crystal display, two electrodes 204 are formed in a glass substrate 200 to drive the liquid crystal molecule 206. Therefore, the rotation angle of liquid crystal molecule in the IPS mode liquid crystal display will be affected once the photolithography process performed in adjacent region changes the distance between electrodes. The different rotation angles will affect the display quality of liquid crystal display.
The typical solution method is to form a tooth-type exposure appearance or to form a mosaic-type exposure appearance in the adjacent region between the masks. However, the two methods require exact alignment; otherwise, the display an image will be affected.